1. Field of the Invention
This invention relates to a silicon carbide semiconductor device having at least one heterojunction.
2. Description of the Prior Art
Silicon carbide (SiC) is a semiconductor material, which has a wide forbidden band gap between 2.2 and to 3.3 electronvolts (eV) as compared with conventional semiconductor materials, such as silicon and gallium arsenide, which have extensively come into use. A silicon carbide semiconductor is thermally, chemically and mechanically stable and also has a great resistance to radiation damage. Both p-type and n-type silicon carbides have good stability, which is rare for wide-gap semiconductors, making these semiconductors useful as semiconductor materials for optoelectronic devices utilizing visible light of short wavelengths, for electronic devices operable at high temperatures or at great electric power, for highly reliable semiconductor devices, for radiation-resistant devices, etc. These semiconductors may be used in an environment where difficulties are encountered with devices made of conventional semiconductor materials, and the range of applications for semiconductor devices using these materials is greatly enlarged.
Silicon carbide has many variant crystal structures (i.e., polytypes or polymorphic forms), which are classified into two types, .alpha.- and .beta.-silicon carbides. .alpha.-silicon carbide of a hexagonal or rhombohedral crystal structure has a forbidden band gap as wide as 2.9 to 3.3 eV, while .beta.-silicon carbide of a cubic crystal structure has a forbidden band gap as narrow as 2. 2 eV. Despite many advantages and capabilities, silicon carbide (including .alpha.- and .beta.-silicon carbides) has not yet been placed in actual use because the technique for growing single crystals of silicon carbide with good reproducibility, which is required for commercially producing high-quality silicon carbide wafers having a large surface area has not yet been developed.
In recent years, the inventors have developed a process for growing large-sized high-quality single crystals of .beta.-silicon carbide on a single-crystal substrate of silicon by the chemical vapor deposition (CVD) technique and filed a Japanese Patent Application No. 58-76842 (76842/1983) which corresponds to U.S. Pat. No. 4,623,425. This process (referred to as a successive two step CVD technique) includes the steps of growing a thin film of silicon carbide on a silicon substrate by the CVD method at a low temperature and then growing a single-crystal film of silicon carbide on the said thin film by the CVD method at a higher temperature. Results for the application of this process have also been reported in the Journal of Crystal Growth, 70, 1984.
Also, another process for growing large-sized single crystals of .beta.-silicon carbide by the carbonization CVD technique is disclosed in Applied Physics Letters, 42(5), 1 Mar. 1983. Moreover, the inventors have devised a process for growing single crystals of .alpha.-silicon carbide and filed a Japanese Patent Application No. 58-246512 (246512/1983) which corresponds to U.S. patent application Ser. No. 683,651 filed Dec. 19, 1984 and now U.S. Pat. No. 5,037,502. At the present time, these techniques make it possible to produce large-sized high-quality single crystals of .alpha.- and .beta.-silicon carbide, while controlling the polytype, the concentration of impurities, the size, the shape or similar characteristics or single crystals.
On the other hand, in the present semiconductor industry, various semiconductor devices such as diodes, transistors, integrated circuits, large-scale integrated circuits, light-emitting diodes, semiconductor lasers, solar cells, and charge coupled devices, which are made of silicon or a compound semiconductor material such as gallium arsenide or gallium phosphide, have extensively come into use. Many of these semiconductor devices have at least one heterojunction composed of semiconductors with different forbidden band gaps, so that the efficiency of injection of carriers such as electrons or holes, the luminous efficiency of light-emitting devices, and the photoelectric efficiency of photodetectors can be improved so that excellent operating characteristics can be attained.
For silicon carbide semiconductor devices, it is possible to improve the operating characteristics of transistors, the luminous efficiency of light-emitting devices, and the photoelectric efficiency of photodetectors by utilizing the heterojunction composed of silicon carbide and a semiconductor with a forbidden band gap different from the band gap of the silicon carbide. It is also possible to develop a semiconductor device which can accomplish novel functions. However, in general, if semiconductor materials other than silicon carbide are used as the above-mentioned semiconductor with a different forbidden band gap, crystal defects will occur at the heterojunction due to the difference between the sizes of their crystal lattices (i.e., lattice mismatches). Moreover, because the heterojunction is composed of silicon carbide and another semiconductor material, the region near the heterojunction is contaminated by a small amount of impurities, so that a deterioration of the device characteristics is caused.